1. Field of the Invention
The present invention relates to an improved method of determining, while drilling in the earth with a drill bit, the positions of geologic formations in the earth. More particularly, it relates to a method for improving the quality of the acquired data.
2. Description of the Related Art
Conventional reflection seismology utilizes surface sources and receivers to detect reflections from subsurface impedance contrasts. The obtained image often suffers in spatial accuracy, resolution and coherence due to the long travel paths between source, reflector, and receiver. In particular, due to the two way passage of seismic signals through a highly absorptive near surface weathered layer with a low, laterally varying velocity, subsurface images are poor quality. To overcome this difficulty, a technique commonly known as vertical seismic profiling (VSP) was developed to image the subsurface in the vicinity of a borehole. With VSP, a surface seismic source is used and signals are received at a single downhole receiver or an array of downhole receivers. This is repeated for different depths of the receiver (or receiver array). In offset VSP, a plurality of spaced apart sources are sequentially activated, enabling imaging of a larger range of distances than is possible with a single source
During drilling operations, the drillstring undergoes continuous vibrations. The sensors used for making measurements indicative of formation parameters are also subject to these vibrations. These vibrations result in the sensor measurements being corrupted by noise. For the purposes of this invention, we distinguish between two types of noise. The first type of noise is that due to the sensor motion itself. This type of noise is particularly severe for nuclear magnetic resonance (NMR) measurements where the region of examination of the NMR sensor is typically no more than a few millimeters in size. With NMR measurements, the nuclear spins in the region of interest are prepolarized by a static magnetic field. The nuclear spins are tipped by a pulsed radio frequency (RF) magnetic field, and spin echo signals may be measured by applying a sequence of refocusing pulses. With this arrangement, sensor movement of a few mm results in the signals originating from regions that were either not prepolarized or partially polarized, resulting in low signal levels.
Examples of this type of noise in NMR applications are found in U.S. Pat. No. 5,705,927 to Sezginer et al., U.S. Pat. No. 6,268,726 to Prammer et al., and is U.S. Pat. No. 6,459,263 to Hawkes et al. The Sezginer patent approaches the problem by making the pulse sequence short enough to be tolerant to vibrations of the sensor assembly on the drilling tool. Prammer et al discloses an apparatus and method of NMR acquisition in which motion sensors are used, data are continuously acquired, and after the fact, a decision is made on which data are to be kept. The Hawkes patent discloses the use of motion triggered pulsing, i.e., predicting ahead of time when conditions are likely to be good for acquisition, and acquiring the NMR data based on the predictions.
Prammer includes a summary of the types of drillstring (and tool motion) that occur. These include    (a) Shutdown. This mode is selected anytime the tool detects the presence of metallic casing and/or is on the surface, or detects motion phenomena that make NMR measurements impossible.    (b) Wireline emulation. When no motion is detected, the tool attempts to emulate NMR measurements as typically done by wireline NMR tools.    (c) Normal drilling. During normal drilling conditions, moderate lateral motion is present, which allows for abbreviated NMR measurements.    (d) Whirling. During whirling, lateral motion is violent, but short time windows exist during which the lateral velocity drops to a point, where a porosity-only measurement is possible. The tool identifies these windows and synchronizes the NMR measurement appropriately.    (e) Stick-slip. In this drilling mode, windows exist in which short NMR measurements are possible, interspersed with periods of very high lateral/rotational motion. Again, the tool identifies these windows and synchronizes the NMR measurement appropriately.It is to be noted that the “noise” problem addressed in Sezginer, Prammer and Hawkes are due only to the vibration of the sensor. Other causes of noise are not addressed.
However, many of the commonly used formation evaluation sensors are relatively insensitive to tool motion. These include resistivity sensors. Nuclear sensors such as neutron and gamma ray sensors are somewhat less sensitive, but could be affected to the extent that the dual sensors used may see different standoff and hence may result in improper compensation. Borehole acoustic logging tools are relatively insensitive as long as the tool motion is not so large as to severely affect the formation modes that are excited. Seismic while drilling (SWD) methods would be affected if accelerometers and/or geophones are used for detection of acoustic signals generated elsewhere whereas pressure sensors are relatively insensitive to tool motion.
A second type of noise that occurs in MWD is substantially independent of the motion of the sensor. Examples of these are in acoustic logging and SWD where the drillstring and drillbit vibrations are the source of noise. These could be in the form of body waves through the formation, body waves through the drillstring, and tube waves within the borehole. In SWD, other noises include tube waves generated by the seismic source and noise caused by flow of the drilling mud. U.S. Pat. No. 6,237,404 to Crary et al. recognizes the fact that there are many natural pauses during rotary drilling operations where a portion of the drill string remains stationary. Pauses include drill pipe connections, circulating time, and fishing operations. These pauses are used to obtain formation evaluation measurements that take a long time or measurements that benefit from a quiet environment, as opposed to the naturally noisy drilling environment. Various techniques that are sensitive to the mud flow, weight-on-bit, or motion of the drill string may be used alone or in combination to identify the drilling mode and control the data acquisition sequence. A drawback of the Crary patent is the rather conservative approach in which data acquisition is limited to the pauses in drilling, resulting in data acquisition at a coarse sampling interval corresponding to the length of drill pipe segments. There are situations in which it may be possible to acquire data of adequate quality even outside of the quite intervals defined by the method of Crary.
There is a need for a method of obtaining formation evaluation information in a MWD system that addresses the shortcomings of the aforementioned teachings. Such a method should address noises due to sensor motion as well as noises due to other causes. Such a method should preferably be capable of dealing with a variety of types of noises. The present invention satisfies this need.